The invention relates to a liquid level signal transmitter including magnetic drive means, in which transmitter a switch designed as a reed switch in an inert gas-filled housing is actuated by a magnet connected to a float, and in which transmitter said housing of said reed switch is received in a sleeve closed at one end and is encased in a poured sealing mass.
Such a liquid level signal transmitter is inteded to indicate whether a predetermined level of liquid in a container is reached or not. The switch will correspondingly assume one or the other of two switching positions. A movable float within the interior of the container is lifted by the liquid. When the float arrives at the predetermined liquid level, a switch located outside the liquid container is actuated. Magnetic drive means are known for avoiding the sealing problems associated with mechanical members passing through the container wall. In such magnetic drive means a magnet is connected to the float. That magnet through a flange of non-magnetic material acts on an armature moving with the float movement and actuating switch. Usually, the armature and the switch are mounted in a housing located at the outside of the flange. Considerable problems have resulted therefrom in practice.
Wide temperature fluctuations may occur in the container, the liquid level in which is to be monitored, during the course of chemical processes. Temperatures may change between minimum values well below the freezing point and relatively high temperatures, say for instance between -196 and +180 degrees centigrade. This will result in wide temperature variations of the housing and of the switching mechanism disposed therein. Such variations in the housing temperature may also occur through exterior effects like changes in the exterior temperture.
The air pressure in the housing will decrease when the housing cools. Thus through leaks in the housing, air will be sucked in from the atmosphere. That air contains humidity, a portion of which will condense on the interior wall of the housing and on the surfaces of the parts placed in the housing and freeze on parts whose temperature is below freezing. On rewarming the housing and the air contained therein, above ambient pressure will develop in the housing and result in an exhaust of air from the housing. At least a portion of the moisture condensed from the air, however, will remain within the interior of the housing during this process. This effect, which subsequently will be described as a "breathing" or "pumping effect" is cumulative and after a significant period of operation of the instrument a layer of ice will form sufficient to block the operation of the switch or cause short-circuits.
There have been different attempts at preventing such undesired phenomenon.
Initially it was tried to prevent the entry of outside air into the housing by sealing. Generally, the sealing has not been sufficiently secure to prevent the accumulation of moisture within the housing but only to delay it. It has been shown, for instance, that outside air enters the housing along the cable insulation. A seal of the housing to preclude the entry of air therein due to temperature variations occurring for long periods of operation, if ever feasible at all, will exceed the tenable cost for technical instruments.
Also, absorption of the moisture entering the housing by a desiccant like silica gel disposed in the housing has been attempted. This, however, will delay accumulation of moisture and ice formation within the interior of the housing only for the period of time required for saturating the silica gel with moisture.
Another alternative attempted has been to heat the housing to prevent condensation of humidity within the housing. Such heating requires a continuous energy consumption. Also, the heating may not be permitted in instruments for use in rooms which may contain an explosive atmoshpere.
A liquid level signal transmitter comprising a discshaped flange or body and a double-armed lever extending normal to the flange and supporting a float with one arm and a magnet with the other arm, said magnet actuating a switch right through the flange, is known for instance through German Patent Specification No. 1,149,077. However, in this known arrangement the switch is situated within a relatively large housing so that even after sealing of that housing the initially described breathing effect will occur.
A liquid level signal transmitter is known in which the electrical switch is a reed switch in an inert gas-filled housing, said switch being located within a cavity formed by a radial bore in the flange (Swiss Patent Specification No. 521,573). In this known arrangement the flange forms a horizontal cover of a housing communicating with the container. A float is guided for vertical movement within the housing. In this known arrangement the float naturally is unbalanced. The float must be buoyant within the liquid to move upwardly towards the cover as the liquid level rises and to bring a magnet affixed to the float within the range of the reed switch. Such a liquid level signal transmitter is unsuited for monitoring the liquid level of low density liquids under high pressures, for example liquefied gases. The cavity within the flange holds a relatively great volume of air in which the connecting terminals of the reed switch are located. For the reasons mentioned and in spite of the cavity being sealed from the outside, moisture can accumulate within the cavity because of the breathing effect upon periodical, large temperature variations, which moisture in the course of time will impair the ability of the signal transmitter to function.
Furthermore, an arrangement including a reed switch located in a vertically disposed, elongate sleeve is known (British Patent Specification No. 1,391,776), said sleeve projecting centrally into a housing connected to the container and being closed at its lower end. A float including a permanent magnet is movably guided along the cylindrical face of the sleeve. The interior of the sleeve is filled by an insulating mass. In this known arrangement the insulating mass serves the purpose to ensure insulation of the central supply line to the reed switch from the metallic sleeve which serves as the return line. Actuation of the reed switch in this known arrangement occurs by vertical movement of the unbalanced float along the sleeve. Thus the float must be buoyant in the liquid to be monitored. This also will preclude an employment of such arrangement in a liquid of low density at high pressure like a liquefied gas.
Through German Offenlegungsschrift No. 26 27 253 a liquid level signal transmitter including magnetic drive means is known in which a float is supported by means of a double-armed lever at the interior face of a flange being designed for vertical mounting at the container and covering an opening in the container wall. A magnet is located at the flange supporting end of the lever. This magnet actuates a reed switch. The reed switch is located within a sleeve closed at its interior end and disposed within a radial bore of the flange. The interior space of the sleeve is filled with a poured sealing mass around the inert gas-filled reed switch housing. There is practically no volume of air in such an arrangement which might cause a breathing effect.
With an unfavorable selection of materials tensions may occur at the reed switch housing due to different thermal expansions in the materials as used. Thereby the switching point of the reed switch may be displaced. Also, faulty switching may occur which may have severe consequences in chemical processes. From the requirement for accommodating the materials in their thermal expansion over wide temperture ranges an undesired restriction on the material selection results.
An object of the present invention is to provide a liquid level signal transmitter of the kind as defined initially and including magnetic drive means, which transmitter will work trouble-free for long periods of operation even with continous periodical temperature variations and which additionally avoids faulty function of the reed switch due to different thermal expansions of the materials employed.
According to the invention, this object is achieved by sealingly retaining the outer end of the reed switch housing facing the open end of the sleeve at a rubber-elastic plate engaging the interior wall of said sleeve, by soldering the connecting wires of the reed switch to the lines of a connecting cable within a space formed in said sleeve intermediate said rubber-elastic plate and said open end of said sleeve and by filling said space with a pourable sealing mass having a low coefficient of thermal expansion, the soldered spots being tightly encased into said pourable sealing mass.